The primary objective of this study is to test whether the administration of supplemental oxygen to infants who develop moderate retinopathy of prematurity (ROP) will result in a reduction of at least one third the number of infants with eyes progressing to threshold ROP. Infants receive continuous pulse oximetry saturation monitoring at one of two specified oxygen levels with weekly measurement of ROP status. The Philadelphia STOP-ROP Clinical Center remains active at the Children's Hospital of Philadelphia as part of a consortium of four intensive care units. Eight patients have been enrolled to date since recruitment began in 1994. One of the four patients enrolled in 1995 was followed in The Children's Hospital intensive care nursery. Participation in this protocol will continue with enrollment of eligible patients. Ophthalmologists will continue ROP screening exams in the nurseries and will alert study personnel of babies who are at risk for developing pre-threshold ROP.